The present invention relates to a merchandiser type refrigerated display case or cabinet used primarily in retail food and supermarket outlets. More specifically, it relates to a device to be used with such cases during the defrosting of the cases by the circulation of ambient air.
The term "refrigerated", in accordance with the present invention is intended to incorporate those cases maintained at a temperature at or in excess of 32.degree. F. such as display cases utilized for display of milk and fresh foods, and those cases maintained below 32.degree. F., such as frozen food cases.
In the operation of all types of refrigerated display cabinets, it is desirable to include a system for automatically defrosting the refrigeration coils. The defrost cycle can be actuated either at set periodic time intervals or when the frost build-up within the system has reached a certain predetermined level. Such systems are typically thermostatically controlled so as to switch from a refrigeration cycle to a defrost cycle of operation. In this manner of operation, it is possible to avoid any significant frost build-up within the display cabinet such that inoperability and spoilage of the stored food products would occur.
There have been three different approaches for defrosting refrigerated display cabinets in this art. These are, utilizing electric resistance heaters; passing a compressed refrigerant gas having a high specific heat through a refrigeration coil; and circulating ambient air through a conduit in which refrigeration coils are positioned. Due to the increased cost of energy, efforts have been made to place more emphasis on the utilization of ambient air defrost systems as an alternative to electrical resistance heaters or compressed refrigerant gas defrost systems.
When ambient air surrounding the refrigerated display case is drawn in and utilized as a source of thermal energy in order to defrost the refrigeration coils, the ambient air after passing through the iced coils can be at a low temperature. In such a condition, the ambient air does not cause thermal shock to the stored products if it should come in contact therewith during a defrost cycle of operation. However, during the terminal portions of the defrost cycle the defrost ambient air is elevated in temperature above the temperature of the air mass surrounding the stored products and hence will cause thermal shock when coming into contact therewith. This problem is particularly acute in the defrosting of open top cases with ambient air.
One approach to alleviating the problem caused by the defrost ambient air coming into contact with the stored refrigerated products is to construct the case or cabinet with guide means which direct the defrost air upwardly over the access opening during a defrost cycle of operation. German Offenlegungsschrift No. 28 04 008, published Aug. 3, 1978 shows a guide plate 29 in FIG. 1 which directs defrost air upward from the normal flow conduit in order to expell the defrost air to the air space above the stored products.
A similar air guide device is set forth in Australian Patent Specification No. 51774/79 wherein an inverted U-shaped channel is used to guide ambient defrost air away from the stored refrigerated products and downwardly toward the outside of the display cabinet.
Another solution to the above stated problem is set forth in U.K. Patent Application No. 2016669A wherein the changing thermal and volumetric differences between the defrost ambient air band and the air mass surrounding the stored products are utilized together with a means to create a negative pressure in the display space so that the defrost air is first permitted into the display space and is thereafter caused to be directed away from the display space without the provision of special guide means.
U.S. Pat. No. 3,324,783, to Hickox, assigned to the same assignee as is the present invention, discloses a restricting slot 18 in an air conduit for a different purpose. In that patent, the restriction slot causes a back pressure which results in a uniform distribution of the propelled air along the longitudinal dimension of the merchandise cabinet so that the air flow is uniform prior to passing through an air directing grid structure. The purpose of the grid structure is to allow the refrigerated air band to flow more uniformly across the open top of the cabinet. There is no provision in this patent for expelling an air band away from the cabinet.
U.S. Pat. No. 3,996,763 to Karashima discloses a number of perforated baffles which permit the volume of refrigerated air inside of the cabinet to be adjusted with respect to the air outside of the cabinet in the food market. U.S. Pat. No. 4,148,197 to Karashima discloses an interior conduit wall flap which is openable to permit the flow of ambient defrost air into a portion of the second air conduit during the defrost cycle.
U.S. Pat. No. 4,120,174 to Johnston discloses an open top refrigerated case in which a greater air flow is employed during the defrost cycle. The defrost air is then ejected in a vertical direction at the front of the case through a specially designed bioriented air grille. The case also contains air flow restriction devices which offer greater flow resistance during the refrigeration air flow direction whereby the flow rate during the defrost mode is greater than during the refrigeration mode. The linear flow rate increase which temporarily occurs as the defrost ambient air band is forced through a venturi-like restriction is not employed to impart higher momentum to the air band in order to eject it from the cabinet since this restriction means produces only a temporary increase in velocity. The defrost air band velocity appears to be constant throughout other portions of the conduit. The momentum of the defrost air is then only elevated above the momentum of the refrigeration cycle air band by reason of the greater resistance to flow in the refrigeration mode.